Create a type for `spec` field of an Endpoint

[ShubhranshuSanjeev]

#[derive(Debug, Clone, Serialize, Deserialize, FromRow)]
pub struct Endpoint {
    pub name: String,
    pub endpoint_type: String,
    pub payload_spec_ref: Option<String>,
    pub config_ref: Option<String>,
    pub spec: serde_json::Value,
    pub retry_policy: Option<serde_json::Value>,
    pub created_at: DateTime<Utc>,
    pub updated_at: DateTime<Utc>,
}